This invention relates to conveyor trolley assemblies. More particularly, this invention relates to trolley assemblies for overhead roller conveyors.
In most poultry processing plants there are overhead conveyor lines including picking lines, eviscerating lines, cut-up lines, and packing lines. The average length of a conveyor line is about 150 m. Running speed is about 13 m/min and usually the lines are operated two shifts a day. A complete overhead conveyor includes a chain, trolley assemblies, and H-attachments and pendants. All the parts are assembled and hung from an I-beam alloy track.
A complete trolley assembly is attached every 0.15 m along the line. In some cases, a complete trolley assembly is attached only every 0.30 m and, in between the trolley assemblies, an H-attachment and pendant are attached. Poultry (chickens, turkeys, etc.) having a weight of only about 3 kg are hung approximately every 0.15 m, although such lines are generally rated at about a 50 kg load per trolley.
The wheels of the trolley assemblies are designed to include ball bearings. In some designs, the outside and the inside races of the ball bearings are made of plastic material. A disadvantage of such designs is that the load which is created on the axle is transferred through the bearing balls which are located at any given moment only on the bottom side of the wheel. This means that most of the load is transferred only through one or two bearing balls. It is well known that a ball creates a very concentrated pressure when pressed between two flat panels. Theoretically, the load is concentrated on a point.
The pressure per given area created by the load is higher than permitted for any known thermoplastic material. As a result, "creep" occurs in the plastic parts. Creep in plastic is a moving of material from one area under a constant pressure to another area under less pressure. When creep increases during rotation, the distance between the inner race and the outer race (i.e., between the stationary and the rotating parts) increases to such a degree that a ball can escape from the wheel. As a result, the wheel either stops rotating immediately or the remaining balls also escape and then the wheel stops rotating. The higher the load, the greater the creep, and the shorter the performance life of the wheel.
There are some designs on the market in which a ball cage is used to prevent the balls from escaping. Such a design provides only a marginal advantage and increases the working life of the wheel by some percentage. Among the disadvantages are that the wheel is more expensive and amount of creep is the same.
Creep occurs all around the outside race (on its inside surface), but only on one side of the inner race (the stationary part). The result is that the inner race is no longer round in shape and therefore it becomes easier for the wheel to slip over the I-beam than to rotate on it.
An additional disadvantage of wheels designed with ball bearings is the high cross-section of the outer race (i.e., the wheel itself). A high cross-section is required because both the inner and outer surfaces of the outside race must be smooth. The inner surface, because of the bearing balls, and the outer surface, because it has to roll against the I-beam. This high cross-section poses significant disadvantages:
(1) A very long thermoplastic injection cycle is required;
(2) A substantial amount of thermoplastic material is needed;
(3) A substantial amount of shrinkage and deformation occurs creating lower quality plastic properties (increasing creep and outside abrasion of the wheel); and
(4) A complicated assembly process for the ball bearings is required.
The bracket of the trolley assembly is generally drop forged and coated to prevent rusting. The trolley bracket is designed to be strong enough to hold a load of about 50 kg per bracket for at least 2 years of operation at 2 shifts per day. The weight of the trolley bracket is approximately 225 gr. The trolley bracket has a coating to prevent rust when working in a processing plant having a very high humidity level. Nonetheless, a substantial number of trolley brackets rust despite the fact that all the conveyor parts are replaced every one to two years.
Due to trolley assembly requirements, the thickness of the pendant should be 6 mm. The pendant is made of steel and is also plated to prevent rust. Consequently, the pendant is stronger than needed and weighs about 188 gr. Similarly, the H-attachment is drop forged or made of sheet metal and is coated to prevent rust. Weight of the steel H-attachment is approximately 89 gr.